In a digital camera, an image is captured using a digital image sensor, such as a charge coupled device (CCD) or CMOS sensor formed in an integrated circuit. Signals are then output from the image sensor to form digital data representing the captured image. The image data must be stored in order for the captured image to be reproduced by display on a viewing screen, printed on a computer printer, or the like. Presently, most digital cameras have rewritable memory built into the camera or provided in a removable storage media card, for example. The use of rewritable memory is advantageous because it allows the user to easily discard a stored image if it does not meet the user's requirement. By discarding the image, the image data is erased from the camera memory, which frees storage space for more further images. A disadvantage of rewritable memory is that it is relatively expensive and does not necessarily provide a permanent record of the image data, like a photographic negative does for conventional photography.
Portable digital cameras for still pictures, for example, generate large amounts of digital data representing images. Each digital image may require up to several megabytes (MB) of data storage, and such storage must be available in the camera. To provide for this type of data storage application, the storage memory should be relatively low in cost for sufficient capacities of around 10 MB to 1 gigabyte (GB). The storage memory should also be low in power consumption (e.g. <<1 Watt) and have relatively rugged physical characteristics to cope with the portable battery powered operating environment. For archival storage, data need only be written to the memory once. Preferably the memory should have a short access time (preferably less than 1 ms) and moderate transfer rate (e.g. 20 Mb/s). Preferably, also, the storage memory should be able to be packaged in an industry standard interface module, such as PCMCIA or Compact Flash card.
One form of storage currently used for application in portable devices such as digital cameras is Flash memory. This meets the desired mechanical robustness, power consumption, transfer, and access rate characteristics mentioned above. However, a major disadvantage is that Flash memory remains relatively expensive ($1.50–$2 per MB). Because of the price it is generally unreasonable to use Flash memory storage as an archive device, thus requiring data to be transferred from it to a secondary archival storage.
Magnetic “hard disc” storage can be used for archival storage, even in portable devices. Miniature hard disc drives are available for the PCMCIA type III form factor, offering capacities of up to 1 GB. However, such disc drives are still relatively expensive ($0.5 per MB), at least partially because of the relatively high fixed cost of the disc controller electronics. Miniature hard drives have other disadvantages when compared to Flash memory, such as lower mechanical robustness, higher power consumption (˜2 to 4W), and relatively long access times (˜10 mS).
Removable optical storage discs can similarly be used, and offer one large advantage compared to hard disc. The removable optical media is very inexpensive, for example of the order of $0.03 per MB for Minidisc media. However in most other respects optical disc storage compares poorly with magnetic hard discs including relatively poor power consumption, mechanical robustness, bulk, and access performance.
Another form of archival storage is described in co-pending U.S. patent application Ser. No. 09/875,356, entitled “Non-Volatile Memory”, the disclosure of which is hereby incorporated herein by reference. The memory system disclosed therein provides high capacity write-once memory at low cost for archival storage. Since the data storage is permanent, however, once image data is stored therein it cannot be erased by the user if the image is not suitable.
It would be desirable to provide a storage system for digital cameras and the like that has advantages of both write once and rewritable media.